Polycarbonate articles have a good impact resistance and transparency, but suffer from a poor resistance to scratching or solvent attack. To remedy this it has been proposed to coat the surface of a polycarbonate article with a thin coating of abrasion resistant material. To obtain good adhesion most of these coatings require the use of a primer layer which is applied to the polycarbonate before the coating operation is performed. While such coatings have adequate adhesion properties for many purposes, they are not generally resistant to immersion in boiling water. Furthermore the use of a primer layer is an additional process step and adds considerably to the cost and complexity of the coating process.
Several coatings have therefore been proposed for direct application to polycarbonate articles:
U.S. Pat. No. 4,60O,649 describes a radiation curable abrasion resistant coating which adheres strongly to basic plastic surfaces (polycarbonate is cited as an example). The coating contains one or more polyacrylates or methacrylates containing at least 2.4 unsaturated groups per molecule and a carboxylic acid which contains acrylic or methacrylic unsaturation. A preferred polyacrylate is trimethylol propane triacrylate. Some diacrylates can also be included in the mixture. Isobornyl acrylate may be present in small amounts to enhance rapid radiation cure, flexibility and to enhance the liquidity of the composition or one of its components. Acrylic acid is a preferred carboxylic acid.
U.S. Pat. No. 4,319,811 and European patent application No. 0194986 both suggest radiation curable coatings formed by co-polymerization of an acrylate with three or more acrylate functional groups per molecule and a monomer containing an N-vinyl imido group. The preferred N-vinyl imido group containing monomer is N-vinyl pyrrolidone. The use of N-vinyl pyrrolidone to provide a coating which adheres well to polycarbonate is further exemplified in G.B. patent applicationNo. 2,082,606A. This patent suggests that acrylates with only two functional groups can be used in conjunction with a cross-linker, and suggests that any N-vinyl derivative of a linear or cyclic secondary amide can be used in place of N-vinyl pyrrolidone.
U.S. Pat. No. 4,199,421 uses a copolymer of acrylate or methacrylate with three or more functional groups per molecule, poly(meth)acrylates of mono or polypentaerythritol being preferred, and either tetrahydrofurfuryl(meth)acrylatr ethyl carbitol(meth)acrylate to form a radiation cured abrasion resistant coating. U.S. Pat. No. 4,291,097 says that while such coatings have an excellent abrasion resistance and surface smoothness, they are likely to be readily staticized so as to attract and deposit dust or the like and have been difficult to dye. U.S. Pat. No. 4,291,097 suggests that the further addition of a phosphate type monomer and an ethanolamine compound can substantially improve the antistaticity and dyeability of the coating while maintaining the desirable abrasion resistance and surface smoothness. The coatings are apparently suitable for polycarbonate and the hardened coating is said to be tintable by almost all types of dyes. The examples use a dye bath at 90.degree. C. for 1 hour which may be due to the coating's poor resistance to immersion in boiling the coating water. In any case 1 hour is a relatively long exposure to the tinting bath.
G.B. patent application No. 2,144,345A suggests that the composition of U.S. Pat. No. 4,319,811 can be rendered hard and yet still tintable by curing to achieve between 20 and 80% cross-linking. In fact the tintable coating achieved by this technique is not hard enough for practical applications and must be subjected to a second curing step after tinting. This is extremely inconvenient.
None of the prior art abrasion resistant coatings has been able to combine the desirable features of radiation curing, direct application to polycarbonate, adhesion sufficient to withstand prolonged exposure to boiling water and tintability when in the fully hardened state.
Throughout this patent specification the term "light tint" is used to denote a lens which transmits between 75% of the incident light and 88% which is the transmittability of an uncoated, untinted polycarbonate lens. The term "strong tint" embraces tinted lenses with a transmission in the range 40 to 75%. "Tintable" means capable of being tinted to give a light or strong tint, as required.